1. Field of the Invention
The present invention relates to detecting the presence of an anomaly in or near an object of electrically conducting material by means of eddy currents induced in the object.
The object can be a plate, such as a support plate or a shell, such as a wall. The electrically conducting material can be carbon steel or stainless steel. Suitably, the anomaly is a longitudinal anomaly, for example a crack or a frame element for supporting the plates that form the hull of a ship. In case of a crack, the crack can be in a vessel wall, a wall of a pipeline or a support plate of a bridge.
2. Description of Related Art
International patent application publication No. 95/00 840 discloses a method of detecting cracks in an object of electrically conducting material. The known method comprises inducing an eddy current into a portion of the object with an abruptly changing magnetic field; while the induced eddy current decays in the object portion, detecting the decay of the induced eddy current; determining with respect to time the derivative of the decay of the induced eddy current; determining a value from the derivative which is representative of the thickness of the portion; determining, by use of a magnetic flux leakage technique, the wall thickness of the object portion; and inferring that a plurality of cracks is present where a reduction in wall thickness is indicated by the derivative of the decay and no reduction in wall thickness is indicated with the magnetic flux technique.
Thus, in the known method, two different techniques are required to determine the presence of a crack.
It is an object of the present invention to provide a method of detecting cracks in an object of electrically conducting material wherein only one inspection technique is used.
To this end the present invention provides a method of detecting the presence of an anomaly in or near an object of electrically conducting material using a probe, which probe comprises a transmitter for inducing eddy currents in the object in a footprint area that is not rotational symmetric, and a receiver system for providing a signal indicative of the strength of an electromagnetic field or of changes of the strength of an electromagnetic field, which method comprises the steps of:
a) selecting a set of points on the near surface of the object which are to be inspected;
b) selecting a first inspection point from the set, and selecting a first direction and a second direction that differs from the first direction;
c) positioning the probe at the selected inspection point, activating the transmitter to induce eddy currents in the object such that the footprint is directed in the first direction by activating the transmitter, and determining a characteristic value "PHgr"1 of the electromagnetic field generated by the eddy currents;
d) activating the transmitter to induce eddy currents in the object such that the footprint is directed in the second direction by activating the transmitter, and determining a characteristic value "PHgr"2 of the electromagnetic field generated by the eddy currents;
e) selecting a next inspection point from the set and repeating steps c) and d) until all inspection points have had their turn; and
f) inferring that an anomaly is present at an inspection point if a combination of the characteristic values "PHgr"1 and "PHgr"2 pertaining to the first and second direction deviates significantly from a norm.